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A rough-idling car can be caused by any number of issues. While this might not seem like a pressing matter, a rough idle can signal a problem with the car's engine. Repairing engine problems can mean a significant expense, so it's best to have a rough-idling car checked out by a qualified mechanic as soon as you notice the car idling rough.Engine Misfiring
A misfiring engine is among the most common causes of a rough idle. You may notice your car idling roughly when it's in "Park" or when you're driving it and stop at a stoplight. A rough idle can be caused by damaged spark plugs or spark plug wires or a burned-out valve. It's important to address the cause as soon as possible to avoid potentially damaging the engine. Engine repairs are almost always expensive.Diagnostic Testing
There are numerous tests your mechanic can perform to find the cause of a rough idle. Most of these tests involve checking your car's internal system. Diagnostic tests for rough idling may include a check of your car's ignition wires to determine if they are damaged in any way, examining the vacuum hoses for any splits or tears, the carburetor for any flooding and a cylinder compression test.Solution
If the cause of the rough idle is an engine misfire, a mechanic may start by replacing the spark plugs or fixing a vacuum leak. If the issue stems from a clogged fuel injector, the mechanic may need to clean or replace the fuel injector, depending on the extent of the damage. You may need to replace the PCV valve, if your car is idling roughly after it warms up, according to the Mister Fix It website.Considerations
Don't wait too long after noticing a rough idle to get your car checked out by a mechanic. There are a few diagnostic tests you can do yourself and attempt to determine the problem, however, unless you're a mechanic or have some mechanical capabilities, a professional mechanic will be needed to make the repair. The longer you wait, the worse the problem could be in the end.

The Causes of Rough Idling in Cars

Your car has been running great for years, but recently it has developed a rough idle. The tachometer needle jumps erratically, and the engine surges and wanes. There are some definite causes for the rough idling, and the problems may be simple to repair. Not correcting this problem now may lead to serious engine damage later, leaving you stranded by the side of the road.Vacuum Hoses
A loose or broken vacuum hose causes rough idling, and is the easiest repair of all. This is a very common problem since hoses wear out over time. Look in your engine compartment, and examine the hoses carefully for any signs of abrasion, breaking or loosening. Replace any damaged or broken sections of hose, available at most auto parts stores, and your rough idling will more than likely disappear.Clogged Filters
Clogged filters are another common cause of rough idling. Inspect your air filter. If dirty, replace it. Also, a clogged fuel filter causes rough idling. It's hard to tell if a fuel filter is clogged or not, so a good rule of thumb is to change it at every general tune-up.Electrical Components
Bad spark plugs, bad spark plug wires and a bad distributor cap are other common causes of rough idling. If it has been a while since you changed these components, do so. Most auto service centers can do this for you if you need technical expertise.Timing Belt
A timing belt jumping a tooth is also a cause of rough idling. This throws off cam timing, setting in motion a series of events that prevent your engine from achieving proper timing. This is a more in-depth repair and, if needed, should be referred to a qualified auto mechanic.

How Do I Fix a Car Idle Problem?

A car idling problem can cause intermittent stalling and make it difficult for your engine to operate properly. A low idle may cause the IAC valve, or idle air control valve, to overcompensate for an improperly adjusted throttle. Of course, your IAC valve might be the problem itself. Before you can know what you need to fix, you should troubleshoot the problem. Fixing your idle problem can normally be done within an hour.

1. Open the hood of your car and locate the idle air control valve on the backside of the engine on the throttle body assembly.
2. Unplug the electrical connector running to the valve.
3. Unscrew the screws that hold the idle air control valve to the throttle body assembly and pull the valve off the throttle body.
4. Turn the valve over and look at the sensor wires. If there is any dirt or debris in the sensor, then spray the sensor wires liberally with electronic parts cleaner.
5. Set the dial on your voltmeter to "OHMs".
6. Touch the red lead of your voltmeter to one of the terminals on the end of the sensor and the black lead to the other terminal on the other end of the sensor.
7. Check the reading on the voltmeter. The reading should be "0" but a variance of ".05" is acceptable. The numbers represent the continuity in the sensor. The lower the number, the better.
8. Reinstall the valve.
9. Start the engine.
10.Turn the nut on the throttle cable at the throttle body clockwise to tighten the cable and increase the idle to 800 RPM. If the idle fluctuates, you may need to replace the IAC valve. However, your idle should hold steady if the valve's continuity in step 7 was within specifications.